Efficiency, however, is only one aspect in evaluating QFC performance. Because 637 nm to telecom conversion requires a pump at a shorter wavelength than the output, telecom noise photons can potentially be generated by spontaneous down conversion, a known issue in PPLN devices [1]}. There may, however, be an additional advantage to resonant devices which suppress undesired frequency conversion processes by a combination of phase matching and resonance. Optimal waveguide-resonator coupling and efficient off-chip coupling, while theoretically straightforward, pose engineering challenges. Perhaps the most critical challenge, however, will be achieving triple resonance from a precise defect optical transition frequency to a target telecom frequency. This will require further advancement in multiple independent, or quasi-independent resonator tuning mechanisms [2]}, [3]}. Once achieved, the impact of triply-resonant devices would extend to wavelength multiplexing for quantum networks and the elimination of emitter wavelength inhomogeneity.
